Hydrocarbon polymers generally fall into two broad classes, thermoplastic and thermosetting resins. Thermoplastic resins may be readily worked by heating the polymer up to its softening point or melting point. They may then be processed by such deformation methods as vacuum forming, extrusion of a melt, compression molding, etc.
The thermoset resins can generally not be reworked once they have hardened. In general, thermoset resins owe their unique properties to covalent crosslinks between polymer molecules. The crosslinks may be introduced by interaction of various monomers such as copolymerization of styrene in the presence of smaller amounts of divinyl benzene or the reaction of epoxy type resins with polyamines.
Uncured elastomers such as natural rubber and butyl rubber are thermoplastic. They may, however, be crosslinked or vulcanized by the use of sulfur and accelerators which react with the carbon of the unsaturated bonds in the polymer molecules to form in effect a thermoset product which can no longer be fabricated or worked except by machining or similar techniques. The vulcanized polymers have found wide utility because of the significant improvement in physical properties by crosslinking. Natural rubber, for example, may be crosslinked or vulcanized by the use of sulfur which reacts with the carbon of the unsaturated bonds in the polymer molecule to form a bridge between two molecules so that one polymer molecule is covalently bonded to the second molecule. If sufficient crosslinks of this type occur, all molecules are joined in a single giant molecule. Once crosslinked, the polymer is intractable and can no longer be fabricated except possibly by machine. It has, however, significantly improved physical properties. Thus, by vulcanizing rubber, elasticity, tear resistance, flexibility, thermo-mechanical stability and many other properties are either introduced or improved.
A third class of polymers has recently been developed which, although they are crosslinked, have a softening range of temperatures and may even be dissolved in various solvents. At normal use temperatures, these polymers behave similarly to crosslinked polymers. At elevated temperatures, however, they may be deformed and worked in the same manner as thermoplastic resins. Such polymers are said to be physically crosslinked. An example of such materials is ionic hydrocarbon polymers (ionomers). These products owe their unique properties to the fact that crosslinking is accomplished by ionic rather than covalent bonding between molecules of the polymer.
These ionic polymers or ionomers may be readily prepared by a variety of techniques using numerous homo-, co-, and terpolymers as backbones. However, while all ionomers have several obvious advantages, one disadvantage to all is the increased difficulty in processability as compared to similar polymers having the same backbone but without ionomeric crosslinkages.
Polymers containing modest amounts of ionic groups have been commercially available. For the most part, these polymers contain quantities of ionic groups in amounts which vary from about 0.2 mole percent up to about 15 mole percent. In other words, the average ionic group contents of these polymers have generally been about 2 per 1000 monomer repeat units up to about 150 per 1000 monomer repeat units. While those ionomers containing such a level of ionic groups are well known, there has existed some major limitations concerning the nature of the ionic moiety. If the ionic group is a sulfonic acid or a carboxylic acid, it has been previously observed that such polymers cannot be readily fabricated if the acid groups are substantially neutralized. Hence, the presence of about 10 to about 20% of the free acid is required to permit processability of the product.
Naturally, this limitation has precluded the exploitation of those fully neutralized ionomers. Yet, the fully neutralized systems are precisely those which confer the greatest advantage in terms of physical properties and which also are extremely convenient to prepare. Thus, the availability of suitable technology which makes viable the melt processability of such systems is regarded as a significant advance.
It has been demonstrated that certain sulfonated elastomers could be suitably plasticized by addition of selected volatile or high melting, relatively polar species. This plasticization technique disrupts the ionic association and permits fabrication of such elastomers at elevated temperatures. See U.S. application Ser. No. 383,350, now U.S. Pat. No. 3,847,854 filed on July 27, 1973 in the names of N. H. Canter and D. J. Buckley, incorporated herein by reference. That application teaches that the normally liquid polar plasticizers have no utility as such, since they act as plasticizers over the entire temperature range, including the use temperature of the ionomer. Hence, use of such normally liquid plasticizers results in ionomers which lack those physical characteristics for which ionic domains were introduced into the polymer initially.